Despeckling devices and methods

ABSTRACT

Illuminating coherent or partially coherent light may be directed over an optical fiber and may be despeckled by vibrating the optical fiber or by increasing the number of modes and modal dispersion. An exemplary embodiment is directed to an optical fiber attached to a vibrating device operable to vibrate the optical fiber above a threshold frequency. Another exemplary embodiment is directed to an optical fiber configured to have a refractive index profile operable to increase the number of modes and modal dispersion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional conversion of, and thus claimspriority to, U.S. Provisional Patent Application No. 61/505,217,entitled “Monitoring and control of laser module for projectors,” filedJul. 7, 2011, and U.S. Provisional Patent Application No. 61/505,220,entitled “High power frequency diverse lasers for projection display,”filed Jul. 7, 2011, and U.S. Provisional Patent Application No.61/505,211, entitled “Method and apparatus for speckle reduction,” filedJul. 7, 2011, and U.S. Provisional Patent Application No. 61/505,455,entitled “De-speckling fiber,” filed Jul. 7, 2011 all of which areincorporated herein by reference in its entirety.

TECHNICAL FIELD

Generally, this disclosure generally relates to lasers, and moreparticularly relates to the laser systems.

BACKGROUND

Light Amplification by Stimulated Emission of Radiation, also known aslaser, refers to the emission of light from excited atoms. Laser lightsources are narrow-band light sources operable to provide colored light.Laser light sources may be desirable because of their long lives, highintensity, and superior collimation. Lasers have a variety ofapplications, such as laser engraving, laser bonding, laser pointers,photolithography, lighting, LIDAR, surveying, barcode reading, and lasersurgery.

BRIEF SUMMARY

An exemplary embodiment is directed to a laser system comprising a laserlight source operable to output illuminating light, a display deviceoperable to receive the illuminating light, and an optical fiberoperable directed the illuminating light from the laser light source tothe display device. The disclosed laser system also includes a vibrationdevice attached to the optical fiber, wherein the vibration device isoperable to vibrate at a frequency greater than a threshold frequency soas to reduce speckle.

Another exemplary embodiment is directed to a method of providingillumination. The exemplary method includes outputting illuminatinglight from a laser light source, directing the illuminating lightthrough an optical fiber to a display device, and vibrating the opticalfiber at a frequency greater than a threshold frequency so as to reducespeckle.

Another exemplary embodiment is directed to a laser system comprising alaser light source operable to output illuminating light, a displaydevice operable to receive the illuminating light, and an optical fiberoperable to direct the illuminating light from the laser light source tothe display device. The optical fiber has a core extending along acentral longitudinal axis and a cladding layer covering the core, thecore having a refractive index profile defined from the centrallongitudinal axis to a longitudinal edge, the refractive index profilecomprising a first refractive index at the longitudinal edge that isgreater than a second refractive index at the longitudinal axis.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an exemplary laser sub-module, inaccordance with the present disclosure;

FIG. 2 is a schematic diagram showing an exemplary architecture of alaser system, in accordance with the present disclosure;

FIG. 3 is a schematic diagram showing an exemplary architecture of lasermodule having sub-modules for a color range, in accordance with thepresent disclosure;

FIG. 4 is a schematic diagram showing an exemplary embodiment of a lasersystem, in accordance with the present disclosure;

FIG. 5 is a schematic diagram showing an exemplary vibration deviceoperable to reduce speckling, in accordance with the present disclosure;

FIG. 6 is a schematic diagram showing a convention step index fiber, inaccordance with the present disclosure;

FIG. 7 is a schematic diagram showing a convention gradient index fiber,in accordance with the present disclosure; and

FIG. 8 is a schematic diagram showing a despeckling fiber, in accordancewith the present disclosure.

DETAILED DESCRIPTION

As disclosed in U.S. Pat. No. RE42,251, which is incorporated byreference herein, business projectors may use only 3-6 W to power eachcolor (red, green, and blue) because the screen size is fairly small(5-10 feet). Cinema projectors, on the other hand, may provide 20-500 Wto power each color depending on screen size (20-80+ feet) to providesufficient light to overcome systems losses incurred, such as the lossesinvolved in 3D cinematic display systems. 3D movies may use more powerthan 2D movies, and thus, there is a need for 30-500+W to power eachcolor for digital projection displays.

With this the large power requirement, there is a need for projectorsystems with alternative light sources with higher output performancefor cinema projection display. Lasers are one light source technology toconsider. The use of coherent or partially coherent light sources canhave advantages over standard incoherent sources (lamps) in projectiondisplay. In some embodiments, lasers may allow for higher brightness,better efficiency, require less maintenance, and have larger colorgamuts. A general review of lasers for all display applications isprovided in Kishore V. Chellappan et al., Laser-based displays: areview, Applied Optics, Volume 49, Number 25, pp F79-F98 (2010), whichis hereby incorporated by reference. According to an embodiment,exemplary efficient lasers include direct emitting lasers such assemiconductor (edge emitting) diode lasers, and vertical cavity surfaceemitting lasers (VCSELs). In an embodiment, these may have anywhere from5-70% efficiency in converting electrical power to light power. Theyalso can be very reliable lasting for tens of thousands of hours versus2000 to 5000 hours for most projector lamps.

In an embodiment, suitable laser diodes may include single individualemitters or bars (stack of edge emitters) and may have red, green andblue wavelengths. Red may be defined as light having a wavelengthbetween 615-670 nm; green may be defined as light having a wavelengthbetween 515-550 nm, and blue may be defined as light having a wavelengthbetween 445-475 nm. These colors roughly allow for all the colors neededto cover the Digital Cinema requirements. It should be appreciated thatother colors, or additional colors may be used (e.g., yellow) toincrease the color gamut.

According to an aspect of the present disclosure, improved brightnessand other performance factors may be achieved by using digitalprojectors and displays with laser modules. High power lasers (e.g., >20W per color) with small etendue may be available and useful for cinemaprojection applications, but using conventional laser devices may causeperformance degradation of these systems due to the issues of viewermetamerism and speckle. In some embodiments, the laser modules of thepresent disclosure may be controlled and monitored as separate unitsfrom the digital projectors to optimize performance and minimizeperformance degradation. According to another aspect of the presentdisclosure, desirable performance of laser-based projectors and displaysmay be achieved using fiber configured to reduce speckle.

Metamerism refers to different viewers perceiving the same lightcondition as different colors. This may particularly be an issue fornarrow frequency band primary light sources. In an embodiment, byincreasing the bandwidth of the three primary colors, the issue ofobserver metamerism may be reduced. Ideally, the light would becompletely white to minimize different viewers interpolating the samelighting as different colors. See, e.g., Rajeev Ramanath, Minimizingobserver metamerism in display systems, Color Research and Application,Vol. 34, No. 5, pp 367-8 (2009); B. Oicherman et al., Effect of ObserverMetamerism on Colour Matching of Display and Surface Colours, Color Res.& Appl., 33(5): pp. 346-359 (2008); and Sarkar et al, A color matchingexperiment using two displays: design considerations and pilot testresults, Final Program and Proceedings, CGIV 2010 conference, Joensuu,Finland (2010), all of which are herein incorporated by reference.

Speckle may result from interference of the light on the screen ortarget which causes variations in intensity that can be seen by theobserver or instrument and may be an undesirable side effect of usingnarrow bandwidth sources (e.g., lasers). Speckle may be measured bymeasuring the contrast of the light intensity. This is defined as thestandard deviation over the mean of the intensity as disclosed inJacques Gollier, Speckle Measurement Procedure, Conference ProjectorSummit 2010, Las Vegas Nev., May 7, 2010, which is herein incorporatedby reference. The high frequency intensity variations are undesirablefor display or imaging applications. The speckle pattern may createnoticeable undesirable intensity variations across the display. Specklemay be reduced using various approaches.

An approach to reduce speckle involves using moving one or morediffusers to achieve changes to the phase locally to temporal averageout some of the speckle over the observer's/detector's integrationperiod. The diffusers can also be vibrating with an amplitude that islarge enough to cover several diffractive elements to achieve someaveraging as well. See The above discussed exemplary approaches forreducing speckle are disclosed in U.S. Pat. Nos. 5,313,479, 4,035,068,7,585,078, and 7,922,333, all of which are herein incorporated byreference.

Another approach to reduce speckle involves using moving mirrors orphase modulators to achieve the temporal averaging. Generally, adisadvantage of these techniques includes the use of expensive movingparts or phase modulators. The above discussed exemplary approaches forreducing speckle are disclosed in U.S. Pub. App. Nos. 2011/0102748 and2010/0053476, and U.S. Pat. Nos. 4,155,630 and 7,489,714, all of whichare herein incorporated by reference.

Another approach to reduce speckle involves using a large core, long,high numerical aperture (NA) multimode fiber to “decoher” a laser beam.As disclosed in U.S. Pub. App. No. 2009/0168025, which is hereinincorporated by reference, a 12 mm diameter core fiber with an NA of0.65 may be used. This large fiber may provide some reduction in specklebut deleteriously destroys the brightness of the system since theetendue is so very large. Similarly, U.S. Pub. App. No. 2010/0079848disclosed a very long multimode fiber that can have some benefits butreduces the power with absorption. Other approaches are described inJoseph Goodman, Speckle Phenomena in Optics, Ch. 7 (Roberts and Company2006). All references cited in this paragraph are herein incorporated byreference.

Another approach involves dividing the beam up into parts and thenforcing each part to have different path lengths or changes ofpolarization before recombining the beams. Examples of using fiberbundles or splitter/combiners or lenslet arrays include: U.S. Pub. App.Nos. 2005/0008290, 2010/0296065, 2010/0296064, and U.S. Pat. Nos.4,360,372, 6,594,090, 6,895,149, 7,379,651, 7,527,384, 7,719,738. Anexemplary approach uses a lenslet integrator in conjunction with amoving diffuser to reduce speckle. Another exemplary approach uses amoving lenslet array instead of a diffuser to reduce speckle. Theseteachings use expensive fiber bundles or lens arrays or many fibercoupler/splitters to achieve some reduction in speckle. All referencescited in this paragraph are herein incorporated by reference.

Another approach utilizes sources with larger spectral bandwidths. Thiscan be achieved by chirping the drive current, using several lasers ofdifferent wavelengths, or other means. For instance, in a projectiondisplay, a bandwidth of 4-15 nm can improve resulting image quality.Examples are disclosed in U.S. Pat. Nos. 6,445,487 and 7,620,091, andJoseph Goodman, Speckle Phenomena in Optics (Roberts and Company 2006).All references cited in this paragraph are herein incorporated byreference.

Moving the screen is also a possible approach to address the undesirableproblem of speckle. Chapter six of Joseph Goodman's Speckle Phenomena inOptics disclosed a calculated linear shift rate of the screen in x or yor screen rotation (these motions are the plane of the screen with isroughly normal to the projection) in order to average out some of thespeckle during the observer's/detector's time integration period. Bymoving the screen, the light hits different parts of the screen whichthen changes the speckle pattern. If this is done more quickly than thedetector's integration period (for example, the eye is roughly 20Hz)then the detector will see an average of several speckle patterns, whichresults in a lower speckle contrast. U.S. Pat. No. 5,272,473 disclosedthe use of a transducer attached directly to the screen to mechanicallygenerate surface acoustic waves to minimize speckle is taught. U.S. Pat.No. 6,122,023 taught the use of a highly scattering liquid crystal as ascreen and then electrically changing the liquid crystal states toalleviate speckle. Other uses of scattering liquids or diffuser cells toimprove speckle have be described in U.S. Pat. Nos. 6,844,970,7,199,933, 7,244,028, 7,342,719, and U.S. Pub. App. No. 2010/0118397.All references cited in this paragraph are herein incorporated byreference.

Despite the issues of speckle and metamerism, lasers may have theadvantage of being bright as discussed above. They can have a lot ofenergy and a small etendue, which may be determined by multiplying sizeof the beam by the solid angle of the system's entrance pupil. The smalletendue of lasers may allow lasers to be efficiently used as lightsources for a digital projection display in embodiment in which thesmall etendue of lasers is a good match for the small etendue of spatiallight modulators used in the projectors. If the source has a largeretendue than the projector, not all the light will make it through theprojector.

VCSEL are currently available in red or infrared (IR) as arrays ofemitters. However, the single emitters are currently under one watt perdevice. Thus, an architecture using a plurality of laser devices may beused to reach the powers required by projection display, particularlyfor cinema applications. A potential architecture increasing the powerusing VCSEL arrays or laser diodes could involve the use of sub-modules.

FIG. 1 shows an exemplary sub-module 100 comprising a plurality of laseremitters 102 being combined in fiber 104. The laser emitters 102 may bedisposed on sub-mount 106, and the number of laser emitters 102 mayvary. In an exemplary embodiment, 3 to 12 laser emitters 102 areincluded in the sub-module 100. By combining them before putting intothe fiber 104, better brightness can be achieved. An embodiment ofsub-module 100 may be configured as a bar with a plurality of laseremitters 102 that is fiber coupled out or is collimated and has a freespace output of the beam. In such an embodiment, the laser emitters 102may be physically stacked in one direction in close proximity. In anembodiment, 3 to 200 laser emitters may be included in the sub-module100 having a bar configuration.

For VCSEL, a sub-module 100 may be a single array that has either fiberor free space beam out. The sub-module 100 may have a power of 1 to 2 Wto up to about 40 W. An improved lifetime may be achieved by running theindividual laser emitters 102 with lower power than the rated power.This may be referred to as derating the laser emitters 102.

FIG. 2 is a schematic diagram showing an exemplary laser system 200comprising high output laser modules 220, 230, and 240 for differentcolors. In an embodiment each of the laser modules 220, 230, and 240includes a plurality of sub-modules 210. The number of sub-modules 210in the laser modules 220, 230, and 240 may vary. In an embodiment, thenumber of sub-modules 210 may range between 2 to 50, with eachsub-module 210 being similar to the sub-module 100 and comprising aplurality of single laser emitters (not shown). The single emitters maybe laser diodes such as edge emitters, VCSEL arrays, diode pumped solidstate lasers that are doubled, doubled fiber lasers, and/or directlydoubled VCSEL arrays for each color laser module 220, 230, and 240. Thesub-modules 210 can be combined by free space, waveguides, or by fibercombiners to make one high power laser (e.g., >20 W) for a color. Morespecifically, the light from each sub-module 210 may be combinedtogether by free space combination with mirrors or polarizing beamsplitters, or combined with fiber combiners if coupled to fibers. Thiscan be done for all three colors using different emitters, such as red,green, or blue. The number of laser emitters in each sub-module 210 mayvary. In an embodiment, number of laser emitters in each sub-module 210may range from 2 to 1000. To compensate for combining losses, thesub-modules 210 may have extra laser emitters to provide extra power fordealing with these losses as well as to improve the lifetime of thelaser by allowing the individual emitters to be derated.

Light from the laser modules 220, 230, and 240 may be input into aprojector 250 to provide illumination in the same manner that a lampwould. The projector 250 may include homogenization optics and use anytechniques disclosed herein or known in the art for reducing speckle.The sub-modules 210 of the laser modules 220, 230, and 240 may be incommunication with a controller module 260 that is operable to control,monitor, or report on the lasers status of the laser system 200. In anembodiment, the controller module 260 is operable to automaticallycontrol the parameters of the system 200 within predetermined ranges aswell as report status and alerts via a network (not shown).

It is to be appreciated that by using sub-modules 210, the overall lasermodule 220, 230, or 240 can achieve higher power (e.g., >20 W) per colorand better, cheaper and larger wavelength diversity. Each sub-module 210may have its own drive electronics (not shown) and cooling system (notshown) to allow control over wavelength diversity, power, andtemperature control over an entire sub-module 210 or within a sub-module210. The laser modules 220, 230, and 240 may be controlled by separatecontrollers in the controller module 260. The controller may anelectrical interface such as RS232, USB, RS485, Fiber Channel, SCSI,etc, and may use a communication protocol to pass commands to each laserunit.

FIG. 3 is a schematic diagram showing an exemplary architecture 300 oflaser sub-modules 310 for a color range, such as red, green, or blue.The architecture 300 may be implemented for any of the laser modules220, 230, and 240 shown in FIG. 2. In an embodiment, overall colorbalance may be maintained to get to the correct Digital CinemaInitiative (DCI) white point by adjusting the power per color and/oradjusting the color by changing the temperature of the lasers emittersof sub-modules 310. The overall architecture allows for higher power andgreater control of wavelength, power spectral density by allowingindividual adjustment temperature and current of the sub-modules 310. Asshown by FIG. 3, each sub-module 310 inside can have many controls andbe designed to have different wavelengths to help minimize speckle andobserver metamerism.

FIG. 4 is a schematic diagram showing an exemplary embodiment of a lasersystem 400 with laser monitoring and control features. In this exemplaryembodiment, a controller module 460 may be mounted at the top of thelasers 420, 430, and 440, and may have communication buses 470, such asEthernet and RS485. Each of the lasers 420, 430, and 440 may include aplurality of sub-modules similar to sub-modules 210 and 310 shown inFIGS. 2 and 3. In an embodiment, the lasers 420, 430, and 440 andcontroller module 460 may be disposed in a standard 19 inch rack mountor another non standard size. The optical interface with a projector(not shown) may comprise optical fiber 480.

In an embodiment, one of more optical fibers 480 per color may be usedor all three colors could be put into on single fiber 480. The fiber 480may be multimode, single mode, and/or polarization preserving. Photoniccrystal fibers can be polarization maintaining single mode and yet stillhandle the high power with their expanded mode area. By usingpolarization maintaining fiber polarized light could be gotten to theprojector which could be useful in making simpler and cheaper projectorsfor 2D and 3D applications.

In addition, an electrical cable or wire (not shown) may be run with theoptical fibers 480 to the projector. In an embodiment, the cable may beused to inform the lasers 420, 430, and 440 of the status of theprojector, such as on/off/off, or control the lasers 420, 430, and 440,such as turn off/on/idle, change power, etc. The cable or wire may alsobe used to verify that the light is getting to the projector by havingan optical power sensor not shown) installed in the projector laserinterface at time of installation. This interface with the projectorcould be a custom interface with a single or few wires or use a standardinterface such as RS485, USB, Ethernet, GPIB, RS232 or others or acombination of standard interface and single/couple of wires for poweror other things that might be desired. Power may be supplied by thecontroller module 460 to power motors, or vibration devices, which willbe discussed in greater details in another portion of the presentdisclosure.

Interface with user and installer or maintenance provider can includeseveral interfaces, including a front panel that might includeindicators or displays 490 for various status, such as On/Off/IdleStatus, interrupts okay/bad, interrupt reset, power level per laser,hours run, on/off switch, need maintenance of something wrong indicator,etc. In addition, the communications interface 470, such as an Ethernetor IEEE 802.11x, may allow the user a certain level of access to thelaser module functions while denying them access to others (e.g.,setting power). In an embodiment, the standard interface, such as anEthernet, may allow the lasers 420, 430, and 440 to be placed on theinternet or a network. This would allow the laser installer ormaintenance provider to have remote and local access to all the controlparameters over the network. A typical example of maintenance andinstallation use of this interface would be for calibration of thelasers 420, 430, and 440—setting power per color to set white point.When a laser 420, 430, or 440 is installed to a projector in the field(e.g., in a theater), a technical person may measure the power and thewhite color balance out of the projector and then adjust the laserspower output and/or wavelength to set the white balance and overallpower levels to the desired points. Ethernet interface with access toall commands may be protected by various security mechanisms known inthe art. In an embodiment, the interface may be encrypted and/or requirean authorization process required for access the whole range or subsetof the commands. Network connection to the controller module 460 may bethrough wired or wireless connectivity such as Wi-Fi or other wirelessstandards.

Possible commands and queries and actions allowed by the controllermodule 460 may include:

-   -   Interlocks/Interrupts that may be substantially constantly        monitored        -   Cooling failure of laser (e.g., fans, TEC, temperature):            orderly shutdown        -   Laser module container opened during operation—orderly            shutdown        -   Laser module internal enclosure breach—emergency shutdown;            in an embodiment, laser may be cut off such that the laser            light is reduced to an eye safe level in less than 100            milliseconds.        -   Projector interface umbilical conduit damage—Orderly            shutdown        -   Projector enclosure breach—Orderly shutdown    -   Monitor current and temperature at set point—increase of greater        than X indicates need for replacement in next week/month etc    -   Monitor light power that gets to projector    -   Reset interrupts    -   power on, idle, off status    -   set power on/off/idle    -   power of each laser sub-module    -   current of each laser sub-module    -   temperature of each laser sub-module    -   laser module environmental (box) temperature    -   total current of laser module    -   total power of laser module    -   Laser module ID    -   Laser rack installation date from controller    -   Last date controller was calibrated    -   set power of each laser sub-module    -   set current of each laser sub-module    -   set temperature of each laser sub-module    -   set power of laser module    -   set laser module to constant power mode at set point    -   set current of laser module    -   set temperature of laser module    -   set allowable range of temperatures    -   Number of “ON” hours for each laser module    -   Calibration setting of power, temperature points    -   Signal for required maintenance immediately    -   Signal for anticipated maintenance    -   Separate calibration and power levels for 2D and 3D movies

The controller module 460 may be operable to monitor the lasersub-modules as well as the lasers 420, 430, and 440 as a whole to ensuresome of the above parameters are within the require range for operation.Exemplary parameters to be monitored may include various combinations ofpower, temperature, current, safety conditions (interlocks), fan speed,and wavelength of the lasers. For example, in an embodiment, thecontroller module 460 may substantially automatically track power percolor and may track wavelength as well to maintain the correct whitepoint and color calibration as the lasers 420, 430, and 440 age or theenvironment temperature changes. To provide feedback for control, thesesub-modules may have power, temperature, wavelength (spectrum analyzer),airflow, and current sensors. In an embodiment, for most lasersemitters, such as semiconductor diodes, if the current required to keepthe optical power at a constant level increases beyond a certain level(e.g., increases by 5-40%), then the lasers may be anticipated to be ator near the end of life.

It is to be appreciated that the controller module 460 may be configuredto operate in a feedback mode. In an embodiment, the controller module460 may be operable to adjust the operating parameters of the lasersub-modules and/or the lasers 420, 430, and 440 as a whole based on theparameters being monitored. For example, as parts of the sub-modules dieor degrade or even whole sub-modules die or degrade, the output powermay still be maintained by increasing the current to the other elementsor sub-modules in the laser system 400. The controller module 460 may beoperable to keep a record of a number of the laser/system parameters sothat problems could be identified remotely or changes in parameters,such as, current, on hours, interrupts, failures, temperature changes,can be used by the controller module 460 to maintain performance andmake a record of problems. This information as well as status can bemade available to the user/maintenance provider. In an embodiment, bylooking at the current required, the controller module 460 may beoperable to predict the potential failure or the need for maintenance ofthese items.

By allowing access of the controller module 460 over the internet orother network, the controller module 460 may be configured to allowreporting of potential issues or problem to the user and/or themaintenance provider to minimize down time of the laser system 400. Thisreporting can be done over the network as email, text message, phonecalls, or alerts in a monitoring program that resides remotely. Inembodiment, the controller module 460 may be operable to report when thecurrent at constant power has increased and crossed a predeterminedlevel and alert the user and/or maintenance provider of a future failureso that the laser or sub-module can be replaced before failure occurs.The number of “on” hours can also be used as a predictor of failure andused to alert people to potential problems. In an embodiment, thecontroller module 460 may be operable to provide periodic automaticreports on status and function of the lasers (e.g., power, temperature,current, fans, interrupts status, etc.) and projector interfaceinformation (e.g., light power levels, motor status, interrupt status,etc.) to users and/or maintenance providers. Authorized users/providerscan also request or poll the status of the various parameters of thelasers and their sub-modules remotely or while working on/calibratingthe lasers 420, 430, 440 and the projector. It would also be possible torun commands to fix problems or look at the data that the controllermodule 460 keeps to identify potential problems remotely. An example maybe to reset the interrupts and restart the laser.

2D and 3D movies have significantly different light efficiency. Forexample, the standard 2D movie should be shown at 14 ftL. However, inorder to have a 3D movie shown at 14 ftL, the brightness for a 2D movieshown with those setting would be approximately 27 ftL—generally toobright. In addition, the color balance of the projection system maychange with the polarization switching and glasses used for 3D. Thus, inan embodiment, the controller module 460 may have two calibrations forpower and color balance for both 2D and 3D stored. A button or Ethernetcontrol could be used to switch the laser module between 2D movie modeand 3D movie mode.

It is to be appreciated that the freedom to individually configure andcontrol the plurality of sub-modules in the laser systems 200, 300, and400 allows for various techniques to increase spectral diversity in thelaser systems 200, 300, and 400 and reduce speckle. Provided below aresome exemplary embodiments of the techniques for reducing speckle andmetamerism.

EXAMPLE 1

In an embodiment, individual laser emitters 102 may be selected bywavelength to fill sub-modules 210, 310 with certain range ofwavelengths. Laser diodes naturally have a spread of wavelength due tomanufacturing variations. This spread in wavelength of manufactureddevices may follow a Gaussian distribution. By picking amongst thisdistribution, the laser emitters 102 may be categorized into binsdepending on wavelength. In addition, additional and different centerwavelength devices may be designed to increase the wavelength spreadthat can be selected from. In an exemplary embodiment, the wavelengthdistribution of these bins may be 1 to 3 nm wide. A bin may be includedin a plurality of sub-modules 210, 310 or each sub-module 210, 310 maybe made from a single wavelength bin. Since the overall system 200, 400may use many more emitters 102 than what is in a single sub-module 210,310, the bandwidth is statistically very likely to be covered adequatelyand it is acceptable to not know the exact wavelength. Thus, in anembodiment, an approximate measurement of the lasers' wavelength mayacceptable, since binning the emitters/arrays based on a couple of nm isacceptable. A wavelength spread of laser diodes and VCSEL arrays may bearound 1 to 10 nm. However, for both VCSELs and laser diodes, by usinglarger than 2 inch wafers and decreasing the processing control, largervariations in wavelength are possible—10 to 15 nm. By sorting thediodes/VCSEL arrays into wavelength bins, more emitters 102 at the edgesof the wavelength distribution may be chosen to increase the wavelengthspread, which may substantially guarantee a certain spread, and toincrease the power spectral density of the combination laser and thepower uniformity across the wavelength range. In addition, thesub-modules 210, 310 with different wavelengths can be driven at higheror lower powers to make a more uniform power spectral density. UsingVCSEL arrays may be advantageous because as their individual arrays havetypical bandwidths of 0.5 to 4 nm due to thermal disparity across thearray. Thus VCSEL sub-modules 210, 310 may fill a wavelength bandwidthsubstantially evenly. Binning reduces the cost of measuring andseparating the diodes rather than arranging them distinctly bywavelength as proposed by U.S. Pat. No. 6,975,294. By using multiplesub-modules 210, 310, the number of emitters is much larger, theirwavelengths may be changed by setting the temperature of the sub-modules210, 310, and the power density spectrum can be improved by bothwavelength selection and temperature and power control, allowing for acertain wavelength spread and power spectral density.

EXAMPLE 2

In an embodiment, the uniformity of the power across the wavelengthrange may be increased by operating emitters 102 or sub-modules 210, 310at different powers depending on wavelength. Since the sub-modules 210,310 may be configured to have their own cooling and drive electronics,the optical power of the sub-modules 210, 310 may be adjusted bycontrolling the drive current and temperature. Since the sub-modules210, 310 may have their own wavelength range, the power of across thespectrum can be improved inexpensively. The power of the sub-modules210, 310 may be set during calibration to make the power spectraldensity more uniform or shaped as desired (e.g., Gaussian, etc).

EXAMPLE 3

In an embodiment, the power spectral density uniformity across thewavelength range may be improved by determining the number of emitters102 in sub-modules 210, 310 depending on the wavelength range of theemitters. The power of a sub-module 210, 310 increases with the numberof emitters 102.

EXAMPLE 4

As discussed above, the sub-modules 210, 310 may be configured to bephysical and partially thermally separated and have their own coolingand electronics, and it is thus possible to operate the varioussub-modules 210, 310 at distinctly different temperatures. In anembodiment, the temperature influences the power output by theemitter/array 102 and directly determines the wavelength out of thelaser device comprising sub-modules 210, 310. The red emitters/arrays102 have particularly large wavelength dependence on temperature.Because it is possible to increase the temperature range that variousdifferent wavelength emitters/arrays 102 operates, the wavelength rangeof the whole blue, red, or green laser device may be increased over 2Dor 1D arrays of single lasers as described in U.S. Pat. No. 6,975,294,and the power spectrum can be better optimized for greater uniformityacross the wavelength range.

EXAMPLE 5

In an embodiment, the wavelength ranges of emitters 102 may also bechosen to statically minimize observer metamerism. Certain wavelengthsmay have less variation in color perception. More uniform ranges mayhave fewer variations in color perception. So choosing emitterbandwidths and center frequencies to achieve less metamerism may be donewith laser systems 200 and 400. In an embodiment, the sub-modules 210,310 may be configured to have more bandwidth at certain colors (e.g.,blue) to minimize metamerism See, e.g., R. W. G. Hunt, Measuring Colour,Fountain Press (1996), and Fred Billmeyer, et al., Principle of ColorTechnology, John Wiley and Sons (1981), which are incorporated byreference herein.

As discussed above, a way of directing the light from the laser to theprojector, display, or sample to be probed is to use an optical fiber.In an embodiment, the fibers may be single mode or multimode, which maybe cheaper and easier to couple into than a single mode fiber and cansupport much higher energies. The fiber construction can be standard orsome other construction like photonic crystal. The fibers may have corediameters ranging from a couple of microns to about 1.2 mm. Thenumerical apertures of the fibers may be in the range of 0.1 to 0.8.These are relatively inexpensive and come with a number of standardconnection options. While conventional optical fibers may reduce speckleto some extent, they do not reduce the speckle to levels that areacceptable for many applications.

In accordance with the teachings of the present disclosure, it ispossible to reduce the level of speckle by vibrating the fiber atfrequencies higher than the detector can see. For example, if used in anembodiment for eye frequencies, a vibration frequency faster than athreshold frequency may be used, such as 20 Hz. In an embodiment formore advance display systems, the threshold frequency may be higher,such as 60 Hz or 120 Hz. One or more vibration devices can be used alongthe length of the fiber to achieve the time averaging caused by thechange in path lengths of the modes of the fiber. Vibrating the fibermay create local time-varying mechanical stress in the glass of thefiber, inducing local time-varying changes in the glass index. Thevibration may be generated and propagated from any mechanical mechanismsuch as vibration devices used in cell phones or piezo-electric, etc.Thus they can be simple, inexpensive and reliable. In an embodiment, thevibration devices may be located along straight sections of the fiber oron bends. In addition, the fiber may be looped in circles and a deviceattached to several loops may be used.

FIG. 5 is a schematic diagram showing an example of a light source 500that is connected to a device 510 where the light is needed through anoptical fiber 520. For example, the device could be a projector fordisplaying movies or images or any other display device disclosedherein. At least one vibration device 530 may be attached to the opticalfiber 520 by adhesive or mechanical constraints around the standardfiber protective tubing (not shown). The vibration device 530 may remainexposed or may be covered by a protective covering placed over thevibration device 530 or even along the entire fiber 520. In an exemplaryembodiment, armored fiber tubing may be used to enclose the vibrationdevice 530 and the fiber 520. In an embodiment, the vibration device 530may be in communication with a controller 540 that senses speckle and inresponse, activates the vibration device 530. In another embodiment, thevibration device 530 may be in an always-on state. For the portion offiber 520 subjected to the mechanical movement from the vibration device530, the fiber composition may be designed to withstand mechanicalimpact and wear. Techniques to make the fiber more wear resistantinclude adding materials to the fiber composition that do notsignificantly impact the optical transmission performance of the fiber.It to be appreciated that the embodiment shown in FIG. 5 may beimplemented with the laser systems 200 and 400 as well as any othersuitable systems disclosed herein to reduce speckle.

In a laser-based display system, fibers are a convenient way to combineall of these sources and/or sub-modules together as well as to take thefinal output and get the light to the projector. The red, green, andblue lasers light could be sent to the projector using one or more fiberper color or even further combined into 1 or 2 fibers that have allthree colors.

In accordance with the present disclosure, the index profile of theoptical fiber may be designed to despeckle the light without losing toomuch light (making the fiber too long) or increasing the etendue toomuch, thereby minimizing the loss of brightness of the source. In anembodiment, the fiber may have the opposite index profile of aconventional gradient index fiber, with larger refractive index furtheraway from the center of the fiber. This may potentially increase thenumber of modes by increasing the splitting of the light beam, andincreases the modal dispersion by de-phasing the split light beamsquickly. While such a fiber index profile design contradicts that of aconventional optic fiber and would be ineffective for conventionalcommunications, fibers of such an inverse fiber index profile may befabricated by the standard MCVD (modified chemical vapor deposition),PECVD (plasma enhanced chemical vapor deposition, or other CVD processthat is used to make standard gradient index fiber cores. In addition,in an exemplary embodiment, air holes may be defined in a cladding layerof the fiber to lower its index to increase the index contrast andincrease the number of modes that the fiber supports. These twotechniques can be combined to make a very effective despeckling fiber.

FIG. 6 is a schematic diagram showing the index profile of aconventional fiber 600. The conventional fiber 600 may be a standardmultimode fiber or a single mode fiber having step index profile. Asillustrated, the core 602 of the fiber 600 has a radius of a and anindex of n₁. The cladding 604 of the fiber 600 has an index of n₂. Theindex of the core 602 is constant as a function of the radius a as shownin FIG. 2.

FIG. 7 is a schematic diagram showing a conventional gradient indexfiber 700. The V parameter for the gradient index fiber 700 is given byequation (1):

V=(2πa/λ_(o))n₁V(2Δ)   (1)

Where λ_(o) is the wavelength of light, and Δ=(n₁−n₂)/n₁. In the casewhere there are a lot of modes present (V>>1) then the number of modesin the fiber (M) is approximately given by equation (2):

M=(4/π²)V ²   (2)

For gradient index fiber the index of the fiber is varied as a functionof radius. The center is always higher in the middle of the fiber andlower towards the cladding layer. Typically the index of the core (r<a)is written as equation (3):

n ²(r)=n ₂ ²+2n ₁ ²(r/a)^(p)((n ₁ ² −n ₂ ²)/n ₁ ²)   (3)

where p is the grade profile parameter.

FIG. 7 shows the index profiles for different values of the gradeprofile parameter for the conventional gradient index fiber 700. See,e.g., Bahaa E. A. Saleh and Malvin Carl Teich, Fundamentals of Photonic,p. 288 Wiley & Sons (1991), which is herein incorporated by reference.

The advantages of the gradient index fiber 700 is that the index can bechosen so that there are fewer modes propagating, and because the indexis lower towards the cladding 704 (light travels faster there) theprofile can be picked (p=2) so that the higher order modes stay nearlyin phase with the lower order modes (modes that go closer down theaxis). This minimizes modal dispersion and the extent a pulse comesapart into separate modes. Typical comparison of modal dispersionbetween gradient index fiber 700 and step index fiber 600 is for Δ=0.01,n=1.46 modal dispersion is n₁Δ/2c. For step index fiber with the aboveparameters, this results in a dispersion of 24 ns/km. In an optimal p=2gradient index fiber, the dispersion with those parameters is 122 ps/km,which is much better. However, this is undesirable for eliminatingspeckle.

The number of modes for a gradient index fiber 700 depends on the indexprofile. They range from slightly more modes to significantly lessmodes. The expressions for number of mode (M) for p=2 is given isapproximately M=V²/4. This is approximately half the number of modes forthe step index fiber 600.

According to an exemplary embodiment of the present disclosure, amultimode fiber may be configured to increase both the number of modesand modal dispersion to reduce speckle efficiently. Increasing V willincrease the number of modes, and this may be accomplished in a varietyof ways. By using a photonic cladding or otherwise reducing the claddingindex by doping the cladding glass (e.g., Fluorine), A may be increased.In an embodiment, n₁ may be increased with some limitations by dopingthe glass with other materials. In an embodiment, V may be increased byincreasing the core size. However, this reduces the brightness of thelight coming out. In embodiments for projector applications, a radius aof 30 micron (some number of modes) and less than 800 micron (reductionof brightness) may be preferred, with some fiber of up to 2 mm diameterbeing used. In an exemplary, a radius a may be optimized to fall betweenabout 100-600 microns to maintain brightness, handle the large powerneeded, allow for despeckle, and make coupling easy.

Modal dispersion depends in the index difference (larger is better) andthe profile. In an embodiment, by having a profile that contrasts withthat of the gradient index fibers 700, the dispersion may be increased.FIG. 8 is a schematic diagram showing an example of such an indexprofile versus core radius for a despeckling fiber 800. The opticalfiber 800 has a core 802 extending along a central longitudinal axis 810and a cladding layer 804 covering the core 802. The core 802 has arefractive index profile defined from the central longitudinal axis 810to a longitudinal edge 812. The illustrated refractive index profileinclude a first refractive index at the longitudinal edge 812 that isgreater than a second refractive index at the central longitudinal axis810. More particularly, the refract index profile comprises a nadir atthe central longitudinal axis 810 and a zenith at the longitudinal edge812. The illustrated index profile may cause the higher order modes tohave a much longer optical path than the lower order modes and thereforehave large dispersion. Fiber 800 may effectively split the beam intomany more sub-beams (modes) and have larger modal dispersion thanconventional fibers 600 and 700, which are designed for communicationspurposes. Having larger index as the radius a increases, speckling oflight may be reduced while maintaining the brightness, and shorter fibermay be used to result in less loss of light through absorption. In anembodiment, these fibers 800 may allow a length of 1 m to 30 m be usedto achieve 10-50% or larger reduction in speckle contrast in someembodiments. It to be appreciated that the embodiment shown in FIG. 8may be implemented with the laser systems 200 and 400 as well as anyother suitable systems disclosed herein to reduce speckle.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom less than one percent to ten percent and corresponds to, but is notlimited to, component values, angles, et cetera. Such relativity betweenitems ranges between less than one percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1. A laser system comprising: a laser light source operable to outputilluminating light; a display device operable to receive theilluminating light; an optical fiber operable directed the illuminatinglight from the laser light source to the display device; a vibrationdevice attached to the optical fiber, wherein the vibration device isoperable to vibrate at a frequency greater than a threshold frequency soas to reduce speckle.
 2. The laser system of claim 1, wherein thethreshold frequency is about 20 Hz.
 3. The laser system of claim 2,wherein the threshold frequency is about 60 Hz.
 4. The laser system ofclaim 1, further comprises at least one additional vibration deviceattached the optical fiber, the at least one additional vibration deviceoperable to vibrate at a second frequency greater than the thresholdfrequency.
 5. The laser system of claim 1, further comprising acontroller operable to receive information about speckle in theilluminating light at the display device and control the vibrationdevice depending on the information.
 6. The laser system of claim 1,wherein a portion of the optical fiber is coiled, and the vibrationdevice is attached to the portion.
 7. The laser system of claim 1,wherein the optical fiber has a core extending along a centrallongitudinal axis and a cladding layer covering the core, the corehaving a refractive index profile defined from the central longitudinalaxis to a longitudinal edge, the refractive index profile comprising afirst refractive index at the longitudinal edge that is greater than asecond refractive index at the longitudinal axis.
 8. A method ofproviding illumination, comprising: outputting illuminating light from alaser light source; directing the illuminating light through an opticalfiber to a display device; and vibrating the optical fiber at afrequency greater than a threshold frequency so as to reduce speckle. 9.The method of claim 8, further comprising: receiving information aboutspeckle in the illuminating light; and adjusting the vibrating of theoptical fiber according to the information about speckle.
 10. The methodof claim 8, wherein the threshold frequency is about 20 Hz.
 11. Themethod of claim 8, wherein the threshold frequency is about 60 Hz. 12.The method of claim 8, wherein vibrating the optic fiber comprisesvibrating the optic fiber at two or more different longitudinallocations along the optic fiber.
 13. A laser system comprising: a laserlight source operable to output illuminating light; a display deviceoperable to receive the illuminating light; an optical fiber operable todirect the illuminating light from the laser light source to the displaydevice; wherein the optical fiber has a core extending along a centrallongitudinal axis and a cladding layer covering the core, the corehaving a refractive index profile defined from the central longitudinalaxis to a longitudinal edge, the refractive index profile comprising afirst refractive index at the longitudinal edge that is greater than asecond refractive index at the central longitudinal axis.
 14. The lasersystem of claim 13, wherein a radius of the core is defined between thecentral longitudinal axis to the longitudinal edge, the radius beingbetween 30 to 1000 microns.
 15. The laser system of claim 14, whereinthe radius is between 100 to 600 microns.
 16. The laser system of claim13, wherein the cladding layer is doped and comprises a reduced claddingrefractive index.
 17. The laser system of claim 13, wherein the refractindex profile comprises a nadir at the central longitudinal axis and azenith at the longitudinal edge.
 18. The laser system of claim 13,wherein the laser light source comprises a plurality of laser moduleshaving different wavelength bands, each laser module comprising aplurality of sub-modules, and the optical fiber is operable to receivelight from each sub-modules.
 19. The laser system of claim 13, whereinthe layer system further comprises a vibration device attached to theoptical fiber, wherein the vibration device is operable to vibrate at afrequency greater than a threshold frequency.
 20. The laser system ofclaim 13, wherein the display device comprises a projector.